Re: [PATCH] mm: add stack trace when bad rss-counter state is detected

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Jul 23, 2025 at 05:14:19PM +0800, Xuanye Liu wrote:
>
> 在 2025/7/23 17:10, Xuanye Liu 写道:
> > 在 2025/7/23 16:42, David Hildenbrand 写道:
> >> On 23.07.25 10:05, David Hildenbrand wrote:
> >>> On 23.07.25 09:45, Xuanye Liu wrote:
> >>>> 在 2025/7/23 15:31, Kees Cook 写道:
> >>>>> On Wed, Jul 23, 2025 at 03:23:49PM +0800, Xuanye Liu wrote:
> >>>>>> The check_mm() function verifies the correctness of rss counters in
> >>>>>> struct mm_struct. Currently, it only prints an alert when a bad
> >>>>>> rss-counter state is detected, but lacks sufficient context for
> >>>>>> debugging.
> >>>>>>
> >>>>>> This patch adds a dump_stack() call to provide a stack trace when
> >>>>>> the rss-counter state is invalid. This helps developers identify
> >>>>>> where the corrupted mm_struct is being checked and trace the
> >>>>>> underlying cause of the inconsistency.
> >>>>> Why not just convert the pr_alert to a WARN?
> >>>> Good idea! I'll gather more feedback from others and then update to v2.
> >>> Makes sense to me.
> >> After discussion this with Lorenzo off-list, isn't the stack completely misleading/useless in that case?
> >>
> >> Whatever caused the RSS counter mismatch (e.g., unmapped the wrong pages, missed to unmap pages) quite possibly happened in different context, way way earlier.
> >>
> >> Why would you think the stack trace would be of any value when destroying an MM (__mmdrop)?
> >>
> >> Having that said, I really hate these "pr_*("BUG: ...") with passion. Probably we'd want to invoke the panic_on_warn machinery, because something unexpected happened.
> >>
> > The stack trace dumped here may indeed not reflect the root cause ——
> > the actual error could have occurred much earlier, for example during a
> > failed or missing page map/unmap operation.
> > The current stack (e.g., in __mmdrop() or exit_mmap()) is merely part
> > of the cleanup phase.
>
> Dumping the stack still has some chance of helping identify the issue — at the very least, it
> shows which task triggered the check.

The stack will be actively misleading because it's highly likely to be totally
unrelated.

if you want to know the task, just output current->comm  :)

I think it's not only of no value, it's _ACTIVELY_ misleading. So it's
definitely a no to a dump_stack().

I am also not in favour of a WARN_ON() for the same reason.

Really we should be catching these elsewhere.

If you want to send the patch just outputting thet ask then all good.

>
> >
> > Given that, how should we go about identifying the root cause when such an issue occurs?
> >
> > Is there any existing way to trace it more effectively, or could we introduce a new mechanism
> > to monitor and detect these inconsistencies earlier?
> >
> > Let’s brainstorm possible solutions together.
> >
> --
> Thanks,
> Xuanye
>